After the renewal of electric transport fleet of Riga city almost all trams and trolleybuses now are capable of recuperating braking energy back to the overhead supply line. However, due to the segmented power supply topology of the overhead supply network, and due to nonreversible rectifier feeding substations, not always the braking energy is used. If during the braking there is no other vehicle within the same overhead zone, which is consuming energy, then braking energy is still dissipated in brake rheostats. The main subject of this dissertation is estimation of the energy amount, which is lost in the brake rheostats of trams and trolleybuses, and investigation of the energy recovery possibilities by utilizing supercapacitor energy storage systems. In the work two methods for estimation the untapped braking energy reserves are elaborated, which are based on principles of the probability theory and experimentally recorded power diagrams of the electric vehicles. The methods allow not only to calculate the untapped energy, but also determine the amount of energy saved, depending on the energy storage capacity and power capability. Using the methods, the energy reserves for 18 periphery substations of the Riga electric transport network are calculated, and performance of the potential energy storage system to be installed in one of the substations, is analyzed in dependence of the system parameters. A capacitance balancing principle is proposed and investigated as a solution for voltage balancing problem for supercapacitor battery. Within the work, simulation models of the supercapacitor energy storage system for onboard application are developed; verification of the models is done in simulation software and the analysis of the results is given. Several control strategies for the onboard energy storage system are compared.